Noise power generator utilizing secondary emission sources energized by radio frequency means



April 7, 1970 R. c. HERGENROTHER 3,505,512

NOISE POWER GENERATOR UTILIZING SECONDARY EMISSION SOURCES ENERGIZED BY RADIO FREQUENCY MEANS Filed June 50, 1967 P76 1 RADIO 40 44 FREQUENCY 50 DRIVER COOLANT IN n W/id/B 56 i l 52 /54 65 COOLANT OUT 60 INVENTOR RUDOLF C. HERGENROTHER ATTORNEY United States Patent NOISE POWER GENERATOR UTILIZING SEC- ONDARY EMISSION SOURCES ENERGIZED BY RADIO FREQUENCY MEANS Rudolf C. Hergenrother, West Newton, Mass., assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed June 30, 1967, Ser. No. 650,011 Int. Cl. H03!) 29/00 US. Cl. 331-78 4 Claims ABSTRACT OF THE DISCLOSURE A Wide band noise generator utilizing a dynode secondary electron emitter arrangement within an evacuated envelope with the dynodes biased by an external high power radio frequency oscillatory source. Random collisions of secondary electrons produced by primary electrons is the desired characteristic for the generation of high noise content output signals coupled to a collector electrode disposed between the dynodes within the envelope. An external magnetic field parallel to the axis of the envelope aids in focusing the electrons.

BACKGROUND OF THE INVENTION Present day noise sources generally utilize the discharge of an ionizable medium as the means for the generation of wide band white noise. Such noise sources are limited by saturation due to the restricted volume for containment of the ionizable medium and have an output of only a few milliwatts of power. Amplification by further devices such as traveling wave tubes is therefore necessary in order to obtain substantial power for jamming purposes of radar signals or other electronic countermeasures usage. The need therefore exists for higher powered noise generation sources at microwave frequencies to preferably cover a substantially wide band of many octaves.

SUMMARY OF THE INVENTION An electron discharge device of interest in the art is provided by a plurality of dynode or secondary emitter electrodes within a highly evacuated envelope. The electrode surfaces are highly sensitized to have a very low work function and are especially suited for the liberation of secondary electrons upon bombardment by other electrons adjacent to the surfaces. Commonly, interconnection of the electrons to appropriate DC voltage sources will result in a cloud of electrons oscillating back and forth in a potential valley in accordance with well known retarding field principles to produce periodic oscillatory currents and high frequency oscillations. The period of such oscillations is determined by the transit time of the electrons between the electrodes and the movement of the electrons can be conceived as a pendulum movement in that the population of electrons in the tube will be continually regenerated with the final limits being determined solely by the removal of heat generated during operation of the electrodes. An exemplary device of the type under consideration has been referred to as a pendulator as disclosed in United States Patent No. 2,184,910 issued to P. T. Farnsworth on Dec. 20, 1939. Such devices have been employed for the generation of high frequency oscillations and are commonly plagued by excessive noise generated by the large spread of the electron emission velocities of the secondary electron within the envelope. The present invention seeks to take advantage of the disadvantages in the prior art high frequency oscillator devices by coupling the dynode secondary electron emitter structure to a source of high power radio frequency energy and accentuating the random collisions of the electron population for the generation of excessive noise relying solely on the random process of secondary electron production rather than the random process of ionization occurring in prior art gas discharge noise sources. The output of the embodiment of the invention is coupled to a utilization load by means of a coaxial microwave output line and a collector ring electrode disposed within the envelope.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, and a detailed description of a preferred embodiment, will be readily appreciated after consideration of the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of an illustrative embodiment;

FIG. 2 is a schematic diagram of illustrative high radio frequency biasing means;

FIG. 3 is a fragmentary cross-sectional view of a proposed cooling arrangement; and

FIG. 4 is a cross-sectional view of an alternative cooling arrangement.

DESCRIPTION OF AN EMBODIMENT Referring now to the drawings, a preferred form of the noise generator 10 of the invention is shown in FIG. 1 including an evacuated envelope 12 shown for illustrative purposes as being of a metallic composition. Dielectric materials may of course also be utilized for the envelope walls where thermal considerations are not important. Oppositely disposed cold cathode secondary emitter electrodes 14 and 16 are disposed within the envelope 12. The opposing surfaces of the electrodes 14 and 16 are highly sensitized so as to emit a profusion of secondary electrons upon impact by primary electrons as well as other secondary electrons. A common material utilized for the secondary emissive electrodes may "be silver with a thin layer 18 and 20 of cesium oxide deposited thereon. Other typical materials include aluminum or platinum, as well as specially treated alloy material bearing alkaline earth metal oxides.

Electrode 14 has a central aperture 22 providing entrance for a beam 24 of primary electrons emitted from a conventional cathode assembly 26 indirectly heated by a heater 28. The cathode components are considered to be well known and have been purposely omitted for the sake of clairty. A collector ring 30 is disposed between electrodes 14 and 16. The inner conductor 32 of a con ventional coaxial output connector including the outer conductor 34 coupled to the envelope 12 provides means for the coupling of output noise signals from the overall embodiment of the invention. Outer conductor 34 is grounded as indicated by line 36 similarly to one of the electrodes, illustratively electrode 16 as indicated by the line 38. Practically, with the envelope 12 at ground potential simple connections may be made to this memher.

To assist in an understanding of the invention a few explanatory remarks are in order at this juncture. Noise signals generated in the process of electron multiplication have been carefully analyzed in an article entitled A Theory of Noise For Electron Multipliers by W. Shockley and J. R. Pierce, Proceedings of the Institute of Radio Engineers, Vol. 26, No. 3, March 1938. p. 326, for multi-stage devices. The computation for noise is stated to be derived from the following equation:

where I =the mean square noise current of the nth stage;

I:=DC current output of the nth stage; e=electronic charge;

m=gain per stage of multiplier;

M=total gain of multiplier and Af=bandwidth.

In addition, the equation for the mean square shot noise current 1 for a current I from a temperature limited source is given by the equation:

2 i =2em Comparison of equations (1) and (2) indicates that an electron multiplier has introduced an additional factor of Mm-l in the noise current expression. This factor can be increased by increasing M, the total gain, and by decreasing m, the gain per stage. Thus, the use of many stages of secondary electron noise generators to achieve any degree of efficiency presents a rather cumbersome device. The many terminals as well as voltage potential sources for a multi-stage electron multiplier also adds to the disadvantages.

In an attempt to provide a simplified device and retain the advantages of secondary electron emission as a noise source it has been proposed in the art that a two dynode arrangement be employed using extremely high magnetic fields. It was observed, however, that such a device had such a low efiiciency and low power as to be completely impractical.

An analysis of the equations for the noise currents in electron multiplication devices indicates the desirability of utilizing other impingement velocity enhancing means for sustaining the oscillations in secondary electron sources in order that full utilization may be made of the component of the electron frequencies which results in the generation of noise. In accordance with the teachings of the present invention a radio frequency driver source 40 biases electrodes 14 and 16 by means of leads 42 and 44. An exemplary high frequency source comprises a klystron oscillator device, as well as any of the well known triode electron discharge devices for producing oscillatory pulses. In operation, a beam of primary electrons 24 upon i'mpinging of the sensitized surfaces 18 and 20 generates a dense cloud of secondary electrons indicated generally by the numeral 46 which are then oscillated back and forth between the electrodes 14 and 16 to create further emission of secondary electrons by successive impingements to enhance the noise signals. Very little energy from the electrons will therefore be required to sustain the oscillations between the electrodes 14 and 16.

A longitudinal magnetic field parallel to the axis of the envelope 12 is provided by conventional magnetic field producing means 48 in a direction indicated by the arrow 50. A permanent magnet as well as any of the solenoids utilized in type traveling wave type devices may be employed for the magnetic field producing means.

Referring to FIG. 2, another means for the generation of high frequency driving potentials for the biasing of the electrode is indicated as a schematic circuit. A high voltage triode tube 52 is coupled in this circuit with a resistance 54 and condenser 56 connected to the primary winding 58 of a pulse transformer 60. A suitable high power voltage source 62 is controlled by means of tube 52 to alternately charge and discharge the storage condenser 56. The alternating discharge of the current then through the primary winding 58 of transformer 60 results in the generation of radio frequency oscillation pulses in the secondary coil 65 connected across the electrodes 14 and 16. Electrode 1-6 is indicated as being grounded at 64 similar to FIG. 1. It is understood that other circuits exemplary of radio frequency pulse transformers may be employed in the practice of the invention for the generation of the radio frequency signals for the self-oscillation requirement.

A highly efficient noise power generator may be realized utilizing the foregoing illustrative embodiment since full use is made of the noise frequency generation components of the electrons oscillating back and forth within the device. High power noise generation will no longer be limited by the volume of an ionizable medium stored within the prior art devices but simply by the thermal considerations in the generation of the noise signals by random secondary emission. FIG. 3 indicates coolant means including a hollow conduit 66 surrounding the envelope 12. A liquid coolant, such as any of the well known commercially available products including Freon, is introduced at the end indicated by the numeral 68 with egress provided at the end 70. Conventional circulation means cooperate with the respective ingress and egress ends of the coolant conduit.

In FIG. 4 an alternative embodiment is indicated in the form of an electrode 72 defining a plurality of internal passageways 74 for the circulation of a coolant. This electrode may be utilized for one or both of the secondary cold cathode emitter electrodes. In view of the grounding of the electrodes as well as the envelope the circulation of a coolant is feasible.

This completes the description of the noise power generator employing secondary electron emission in a single stage structure with radio frequency energy sources as the means for sustaining the oscillations to thereby take advantage of the inherent noise in electron multiplication. It will be obvious to those skilled in the art that numerous modifications, alterations or variations may be practiced in the illustrated embodiments herein described. Accordingly, it is intended that any modified embodiments be included within the scope and tenor of the invention as defined in the accompanying claims.

What is claimed is:

1. In combination:

an evacuated envelope;

spaced cold cathode sources having sensitized surfaces adapted to emit secondary electrons upon impingement thereon by electrons;

means for generation and focusing of a beam of primary electrons in the region between said sources;

means for establishing a radio frequency alternating voltage potential between said cold cathode sources to enhance the impingement velocity of said primary electrons and substantially increase the generation of noise signals by the random process of secondary electron emission;

and means for coupling said noise signals from said envelope disposed between said cold cathode sources.

2. An electron discharge device comprising:

an evacuated envelope;

a plurality of oppositely disposed electrodes having sensitized surfaces upon impingement thereon by electrons;

means for the generation of a beam of primary electrons along the longitudinal axis of said envelope;

magnetic field producing means extending parallel to the longitudinal axis of said envelope;

means for biasing said electrodes at a radio frequency alternating voltage potential to enhance the impingement velocity of said primary electrons and substantially increase the noise signals generated during secondary electron emission;

and means for coupling said noise signals from said device disposed between said electrodes.

3. A noise power generator comprising:

an evacuated envelope;

a plurality of oppositely disposed electrodes having sensitized surfaces adated to emit secondary electrons upon impingement thereon by electrons;

indirectly heated cathode means for the generation of a beam of primary electrons directed along the longitudinal axis of said envelope;

magnetic field producing means extending parallel to the longitudinal axis of said envelope;

an external source of radio frequency oscillation signals coupled to said electrodes;

and output coupling means imposed between said electrodes.

4. A noise power generator comprising:

an evacuated envelope;

a pair of spaced electrodes having oppositely disposed sensitized surfaces;

said surfaces being adapted to emit secondary electrons upon impingement thereon by electrons;

indirectly heated cathode means for the generation of a beam of primaly electrons in the region between said electrodes;

magnetic field producing means extending parallel to the longitudinal axis of said envelope;

an external source of radio frequency oscillation signals connected to said electrodes to establish a high frequency alternating electric field therebetween;

a collector electrode disposed intermediately to said electrodes;

and a coaxial output line coupled to said collector electrode.

References Cited UNITED STATES PATENTS 3,300,735 1/1967 Badger 332-7 3,072,786 1/1963 Jones et al. 25041.9 3,334,310 8/1967 Hergenrother 33182 3,260,884 7/1966 Shipley et a1. 250-419 3,312,857 4/1967 FarnsWorth 3155 20 JOHN W. HUCKERT, Primary Examiner B. ESTRIN, Assistant Examiner US. Cl. X.R. 

